Technical Article

Designing Intelligent Battery Junction Boxes for Advanced EV Battery Management Systems

June 14, 2023 by Issac Hsu

As electric vehicles become more popular, the challenge for automakers is to reflect true range while making vehicles more affordable. This means making the battery packs lower cost with higher energy densities. Every single watt-hour stored and retrieved from the cells is critical to extending the driving range.

This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.

The main function of a battery management system (BMS) is to monitor cell voltages, pack voltages, and pack current. In addition, due to the high-voltage design of the BMS, insulation resistance measurement between the high-voltage and low-voltage domains is needed to catch defects in the battery structure and protect against hazardous conditions. 


Figure 1. A traditional BMS architecture (a); a BMS architecture with an intelligent battery junction box (BJB) (b). Image used courtesy of Bodo’s Power Systems [PDF]


Figure 1 presents a typical battery management system architecture containing a battery management unit (BMU), a cell supervisor unit (CMU), and a battery junction box (BJB). A BMU typically has a microcontroller (MCU), which manages all of the functions within the battery pack. The traditional BJB is a relay box or a switch box with power contactors that connects the entire battery pack to the load inverter, motor, or battery charger.

Figure 1a shows the traditional BMS. There are no active electronics inside the junction box. All of the measurements in the BJB are measured at the BMU. Wires connect the BJB to the analog-to-digital converter (ADC) terminals.


Figure 2. High-voltage measurements inside the BJB. Image used courtesy of Bodo’s Power Systems [PDF]


Figure 1b shows the intelligent BJB. A dedicated pack monitor inside the box measures all voltages and currents and passes the information to the MCU using simple twisted-pair communication. It helps eliminate wires and cabling harnesses; and improves voltage and current measurements with lower noise.


Voltage, Temperature, and Current Measurement

Figure 2 shows the different high voltages, currents, and temperatures the pack monitor measures inside a BJB enabled by the BQ79731-Q1 battery pack monitor.

  • Voltage:  The high voltage is measured using divided-down resistor strings. These voltage measurements monitor the state of high-voltage components in the system.
  • Temperature:  The temperature measurements monitor the temperature of the shunt resistor so the MCU can apply compensation and the contractors’ temperature to ensure they are not stressed beyond normal operating conditions.
  • Current: The current measurements are based on either a shunt resistor because the currents in an EV can go up to thousands of amperes, the shunt resistance values are extremely small – in the range of 25 µOhms to 50 µOhms or a Hall-effect sensor used to measure the EV current on the high-voltage rail while still being isolated. Its dynamic range is typically limited. Thus, multiple sensors can be in the system to measure the entire range.


Over-Current Fault Detection and Protection

Detection and prevention of an over-current event are required in a BMS to prevent catastrophic damage that can occur to the battery pack in the event of a short circuit, exposed high-voltage terminal, or defective equipment. An over-current circuit integrated into the BJB unit will use the current sense information measured through the shunt resistor, hall-effect sensor, and the battery pack monitor. This measurement is then processed and compared to a threshold inside the battery pack monitor while the same has the capability of signaling an over-current event through dedicated outputs, which will be used to enable a fuse-driver to blow the high-voltage separator (pyro-fuse). Since the reaction time needs to be as fast as possible, a dedicated signal processing path is implemented in the battery pack monitor device.


Voltage and Current Synchronization

Voltage and current synchronization is the time delay to sample the voltage and current between the pack and cell monitors. These measurements are mainly used for calculating the state of charge and state of health through electro-impedance spectroscopy (EIS). Calculating the impedance of the cell by measuring the voltage, current, and power across the cell enables the battery management system to monitor the instantaneous power of the car.

The cell voltage, pack voltage, and pack current must be time synchronized to provide the most accurate power and impedance estimations. Taking samples within a certain time interval is called the synchronization interval. The smaller the synchronization interval, the more precise the power estimate or the impedance estimate. The more accurate the state-of-charge estimation, the more accurate the remaining mileage drivers get.


Synchronization Requirements

Next-generation BMS will require synchronized voltage and current measurements in less than 1 ms, but there are challenges in meeting this requirement.

TI’s battery monitors can maintain a time relationship by issuing an ADC start command to the cell and pack monitors. These battery monitors also support delayed ADC sampling to compensate for the propagation delay when transmitting the ADC start command down the daisy-chain interface.


Remote Device Communication Support

Another benefit of the intelligent BJB is the streamlined data communication by using the versatile daisy-chain interface not just for battery pack and battery cell monitor devices but also remote devices like EEPROM memory or any type of sensors that are placed in the modules with different physical placement in the vehicle. In this case, the pack and monitor device also acts as an interface translator offering I2C or SPI data to be transferred through the daisy-chain interface, eliminating wires and cabling harnesses and, in turn, reducing the overall EV weight.

The massive electrification efforts happening in the automotive industry are driving the need to reduce the complexity of BMS by adding electronics in the junction box while enhancing system safety. A pack monitor can locally measure the voltages before and after the relays and the current through the battery pack. The accuracy improvements in voltage and current measurements will directly result in optimal battery utilization. The BQ79631-Q1 and BQ79731-Q1 from TI can optimize the performance and reduce the future cost of intelligent BJBs by integrating all necessary system functions in a single device. Effective voltage and current synchronization enable precise state-of-health, state-of-charge, and EIS calculations, resulting in optimal battery utilization.

In addition, TI’s BQ79616-Q1 and BQ79718-Q1 battery cell monitor families offer accurate cell voltage and temperature measurements as a part of the CSU implementation, which enables a complete BMS ecosystem.


Additional Resources


This article originally appeared in Bodo’s Power Systems [PDF] magazine

Featured image used courtesy of Adobe Stock